The present invention relates generally to an electrical connector and, more particularly, to a electro-magnetic and radio frequency interference (EMI/RFI) shield for an electrical connector.
The use of a shield in an electrical connector to eliminate RFI and EMI from interfering with signals being carried by the contacts in connectors is well known. U.S. Pat. Nos. 3,521,222; 3,678,445; 4,106,839, 4,239,318 and 4,326,768 disclose annular shields formed of sheet metal with resilient fingers which electrically engage the outer surface of the plug shell and the inner surface of the receptacle shell of the electrical connector.
A newly proposed U.S. Government specification requires for EMI/FRI shielding of an electrical connector to be in the range of 85-90 dB shielding effectiveness. A common EMI/RFI shield that has been used in the past to meet prior Government specifications is similar to that disclosed in the aforementioned U.S. Pat. No. 4,106,839. The shield comprises a stamped and formed metal spring in which fingers extending laterally from an annular band are reversely folded with the outer ends of the fingers resiliently engaging the inside surface of the receptacle shell. The shield is secured to the plug of the connector by forming the band with a U-shaped cross-section which fits tightly over an upstanding annular flange on the plug shell. Such a shield is not satisfactory for meeting the aforementioned specification because of certain design constraints imposed upon the connector. First, the mating force requirements for the plug and receptacle are so low that the normal force of the spring fingers of the shield must be very low, which reduces the conductance of the spring fingers. Second, because of the requirement to use cadmium plating and an outer chromate coating for environmental protection of the connector, the normal force of the shield spring fingers is not enough to break through the chromate coating, thus creating a capacitor effect between the plug and receptacle. Third, the vibration test requirements for the specification are quite severe. During vibration testing, it has been found that the spring fingers of the prior art shield are over-stressed and break. The length of the spring fingers have been made longer to reduce the fatigue stress on the fingers. However, this in turn lowers the normal force even more between the spring fingers and the receptacle shell. Finally, the aforementioned shield has been found to have a shielding effectiveness of only about 35 dB. For the foregoing reasons, the prior art shield will not meet the aforementioned specification.
The aforementioned U.S. Pat. Nos. 4,239,318 and 4,326,768 each discloses an EMI/RFI shield which is in the form of a spring band that is lanced to provide alternating slits which open at opposite edges of the band. Thus, the band has a generally serpentine configuration which is capable of being expanded over the plug shell of the connector. While this form of shield has reduced inductance as compared to the folded spring finger shield of the type disclosed in the aforementioned U.S. Pat. No. 4,106,839, the required normal force of the shield to meet the requirements of the above mentioned Government specification is too great to avoid fatigue failure of the spring material.
U.S. Pat. No. 4,529,259 discloses a combined electrical shield and environmental seal for an electrical connector in which a coil spring is embedded in a central portion of an elastomeric ring. The ring has enlarged portions on opposite sides of the coil spring which serve as environmental seals. The elastomeric ring is relatively soft so that the coil spring constitutes the principal spring member of the composite element. Such a shield does not meet the requirements set forth above, because the high stresses in the spring generated during vibration cause it to fail in fatigue.
U.S. Pat. No. 3,998,512 discloses a connecting device for a digital wristwatch, which is of some interest with respect to the present invention inasmuch as it discloses an elastomeric backup ring for conductors. However, the conductors are in the form of metallic coatings on the surface of an elastomeric strip, which is interposed between the display panel and logic circuit used in the wristwatch. In such application, where miniaturization is critical and only low forces are required, the elastomeric strip undergoes only slight compression. Such a connecting device is totally unsuitable for EMI/RFI shielding where relatively high spring forces are required, which would necessitate substantial compression of the elastomeric strip, that in turn would cause the conductive coating on the strip to become cracked, thereby greatly impairing if not totally destroying the electrical connection between the conductors interconnected by the device.
Accordingly, which is needed and constitutes the principal object of the present invention is an improved EMI/RFI shield which achieves adequate spring force to meet high shielding effectiveness requirements, produces an effective electrical connection between the plug and receptacle shells of a connector even when subjected to high vibration, and yet avoids over stressing of the spring elements of the shield.